Many polymerizable monomeric materials, and particularly vinyl chloride, are being polymerized today on a large scale commercially either in suspension media or in aqueous dispersion or emulsion, i.e., latex form, employing various colloidal suspension agents, emulsifiers or soaps, and/or synthetic detergent-type dispersing agents. In these methods of polymerization, and particularly in suspension polymerization, agitation (usually vigorous) is depended upon to suspend and/or disperse the monomer particles or droplets throughout the reaction media and to maintain such suspension or dispersion during the polymerization reaction and to assist in heat transfer to the reactor cooling surfaces. However, in most instances, polymer particles produced in accordance with these processes are not uniform in size and shape. In subsequent processing of these polymers, such as, for example, polyvinyl chloride (PVC), it is desirable to have uniform size and shape in the polymer particles.
Various polymerization processes and modifications of existing processes have been heretofore proposed to obtain uniform particle size. For example, one very successful method that has been proposed is that described in U.S. Pat. No. 3,620,988. In said method a monomeric material, such as vinyl chloride, of low solubility in water containing a monomer-soluble free radical type catalyst, is suspended as discrete droplets of a desired size in an aqueous medium thickened with a water-insoluble polymeric gellation agent which imparts plastic flow properties to such medium. The polymerization reaction is then carried out using a batch or continuous process under substantially quiescent conditions, that is, in the absence of turbulence or the absence of shearing forces sufficient to deform the suspended droplets of monomer and/or to damage the polymer bead at any stage of conversion. However, when polymerizing vinyl chloride by such a process, the uniform beads of PVC that are formed are normally clear and glassy in nature and do not have the desired porosity.
There are many cases where porous polymer particles or beads are desirable, such as where the polymer, such as PVC, is to be employed in making plastisols and in extrusion operations. Porous beads would also be desirable in the case of PVC where, because of Government regulations, it is necessary to remove substantially all of the unreacted vinyl chloride therefrom. Porous polymer beads or particles would greatly facilitate such removal.
Another real problem in making vinyl resins is polymer buildup on the inner surfaces of the reactor during the polymerization reaction. This is particularly prevalent in the commercial production of vinyl chloride polymers when the same are produced in the form of discrete particles by polymerization in an aqueous suspension system. It should be borne in mind, that the suspension polymerization system is the most practical, from simplicity of operation, and thus, the most economical. When employing such a system, the monomer(s) are maintained in the form of small discrete droplets by the use of suspending agents and agitation. When the reaction is complete, the resultant polymer is washed and dried. These aqueous suspension system polymerization reactions are usually conducted under pressure in metal reactors equipped with baffles and high speed agitators. However, these suspension systems, under many circumstances, are unstable, and during the polymerization reaction, the polymer builds up on the interior surfaces of the polymerization reactor, including the surfaces of the baffles and agitator. Obviously, this polymer buildup must be removed since it results in further formation of polymer buildup on the reactor surfaces which results in a crust that adversely affects heat transfer and contaminates the polymer being produced. Removal of such polymer buildup is difficult and costly and the obvious way of eliminating the polymer buildup is providing a means of polymerization which substantially eliminates its formation in the first place.
Thus a process which will produce vinyl resin particles which are spherical, and porous throughout and which will also substantially eliminate polymer buildup in the reactor, is most desirous.